chore(package): update helmet to version 2.1.1

https://greenkeeper.io/

package.json

@@ -126,7 +126,7 @@
     "dompurify": "0.8.2",
     "express": "4.13.4",
     "extract-text-webpack-plugin": "1.0.1",
-    "helmet": "2.1.0",
+    "helmet": "2.1.1",
     "isomorphic-fetch": "2.2.1",
     "jed": "1.1.0",
     "jsdom": "9.2.0",